Threshold switching in niobate glass — thick-film devices

Thick-film, niobate-glass threshold switches have been made in reducing gas atmospheres at high temperatures (1100–1400°C) using SiO2 and B2O3 as glass formers. Their temperature characteristics are far superior to those of similar vanadate switches previously reported [1,2], and they are stable at low ON currents ( The process consists in pre-firing the printed, powdered glass in air and then reducing in argon/hydrogen gas. While the reducing conditions are not critical and virtually full reduction to tetragonal NbO2 occurs, the pre-firing conditions in air are critical if high-resistance devices are to be obtained. Many devices can be made on a single alumina chip and the results of a statistical analysis are given. The electrical characteristics — measured, chiefly, using a double-pulse technique to examine the change in delay time with pulse separation, ON current, pulse width, etc. — are similar to those previously reported for the vanadate switches [2]. These measurements, together with the results of a microscope and electron-microprobe examination, indicate that a thermal mechanism of switching with filament or channel formation is the most likely. A steady-state computer simulation based on the thermal model indicates the temperatures to be expected during switching, and these are calculated to be less than the NbO2 semiconductor-metal transition temperature of roughly 800°C, for all the cases examined.